The subject invention generally concerns the field of circuitry for compensating attenuators to correct for errors caused by stray capacitance, and specifically concerns novel architecture for such compensating circuitry.
High impedance attenuators are commonly used in modern digital test equipment, such as oscilloscopes, digital multimeters, and the like, to greatly reduce undesirable interaction with (i.e., loading of) a circuit under test. Attenuators also serve to reduce large amplitude signals to prevent overloading of the input signal amplifier (i.e., front end) of the test and measurement instrument.
Typical high impedance attenuators for use with oscilloscopes exhibit a 10:1 attenuation of the input signal. Such attenuators normally employ a resistor-capacitor (R-C) voltage dividing arrangement and an amplifier. In such arrangements variation in the resistances is not a problem because highly accurate resistors are generally available. That is highly accurate DC attenuators are easily realized. However, variation in the capacitance values of the capacitors is much more of a problem. For this reason, those skilled in the attenuator art have traditionally chosen to adjust the capacitors, rather than adjust the resistors. High impedance attenuators are vulnerable to the effects of stray capacitance variation because the capacitors they use are small in value. Variations in circuit board traces of the attenuator, variations in the values of its components, and variations in its own input amplifier, all contribute to changes in high frequency attenuation. Unfortunately, these variations and variation in input capacitance values and stray capacitance adversely affect the operation of these attenuators by distorting the leading edges of signals under test.
Prior art attempts at correcting this problem have met with some success but have generally introduced problems of their own. For example, hand adjustment of variable capacitors is an unreliable procedure. One solution to this problem is to use electronically variable capacitors (varactor diodes), but unfortunately, varactor diodes are inherently non-linear. Other problems may further include the introduction of noise by variable gain amplifiers driving capacitive feedback, frequency-dependent problems, such as phase delay, or problems related to requiring a variable gain amplifier to absorb a significant portion of a high-frequency input current.
What is needed is a relatively inexpensive compensated attenuator circuit, for use in a test and measurement instrument, which does not exhibit, or which reduces the effects of, the undesirable traits listed above.
A high impedance attenuator for use in a test and measurement instrument employs compensation to adjust the low frequency attenuation to match the high frequency attenuation exhibited by the attenuator, rather than attempting to adjust the high frequency attenuation exhibited by the attenuator.
In an alternate embodiment of the invention, compensation to adjust low frequency attenuation is employed in a feedback loop and an opposite resistance is applied in an additional attenuator stage to stabilize the input resistance.
In yet another embodiment of the invention, compensation to adjust low frequency attenuation is employed by means of an R-C time constant of an additional R-C circuit in a feed forward loop. This additional time constant is matched to the R-C time constant of the input R-C network. The input resistance of the attenuator is not changed.